1. Field of the Invention
The invention relates to a steering angle detection device configured to enable the detection of a steering angle of a vehicle over the entire range in which steering is possible, and a method therefor.
2. Description of the Related Art
Steering of a vehicle is typically implemented by transmitting a rotation operation of a steering member such as a steering wheel to a steering mechanism via a steering shaft and changing the orientation of the left and right front steerable wheels by the action of the steering mechanism. The steering angle of the vehicle or orientation of steerable wheels obtained as a result of such steering is information that is necessary for a variety of control operations in the vehicle, such as increase-decrease control of a steering assist force in a power steering apparatus and distribution control of a braking force in an antilock brake system, and steering angle detection devices for detecting the steering angle are installed on many vehicles.
The steering angle of a vehicle can be found by detecting the rotation angle of a shaft that transmits a rotation operation of a steering member, such as the steering shaft, and successively integrating the detection value. Further, in a steering apparatus configured as an electric power steering apparatus, the steering angle can be also found by detecting a rotation angle of a motor for steering assist and successively integrating the detection value. The effect obtained in this case is that no special sensor is required because the detection value of the motor rotation angle is used that is necessary for driving the motor.
Further, a rotation angle detector that outputs a signal showing a plurality of periodic fluctuations in one turn of the object shaft, such as a resolver, is widely used as a shaft angle detector that detects the rotation angle of a steering shaft or a motor angle detector that detects the rotation angle of a motor. A steering shaft can make 4 to 5 turns in the entire steering angle range between the left and right steering limit positions, and a motor for steering assist makes the number of revolutions corresponding to a reduction ratio of a reducer provided for reduction transmission of power to the steering mechanism within the same entire steering angle range.
The detection values of the shaft angle detector and motor angle detector thus become the repetitions of multiple periodic fluctuations within the entire steering angle range, and in a steering angle detection device configured to find the steering angle by integrating these detection value, an absolute steering angle as a steering angle integrated value from the starting point is required. When the vehicle travels, a correct steering angle can be found, for example, by performing the integration by taking a straightforward travel state as a starting point with a zero absolute steering angle. However, the problem arising immediately after the vehicle starts traveling from a stopping state in which the power source was cut off by turning off the key switch is that the absolute steering angle serving as a reference is unclear and the steering angle cannot be determined before a straightforward travel state is assumed and a starting point can be established.
To resolve this problem, a steering angle detection device disclosed in Japanese Patent Application Publication No. 2000-296781 (JP-A-2000-296781) is provided with both the shaft angle detector and the motor angle detector mentioned hereinabove, and the absolute steering angle can be detected by combining the detection values of the two detectors.
This steering angle detection device uses the fact that the detection values of the shaft angle detector and motor angle detector are repetitions of multiple periodic fluctuations within the entire steering angle range, as mentioned hereinabove, and the detection of absolute steering angle is made possible by settings such that the fluctuation periods of the two detectors do not overlap and that identical detection value combinations are not generated in the entire steering angle range.
However, in the steering angle detection device disclosed in JP-A-2000-296781, under specific conditions, the initial steering angle determined by a combination of detection values of the shaft angle detector and motor angle detector sometimes does not correspond to the actual steering angle. As a result, in some cases, an error is generated in the steering angle detection value that uses the initial steering angle as a reference, and this erroneous steering angle detection value impairs the execution of control of various kinds.